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EmbeddedSystemsDesign:AUnifiedHardware/SoftwareIntroduction1Chapter7DigitalCameraExample太原房产网:AUnifiedHardware/SoftwareIntroduction,(c)2000Vahid/Givargis2Outline•Introductiontoasimpledigitalcamera•Designer’sperspective•Requirementsspecification•Design–FourimplementationsEmbeddedSystemsDesign:AUnifiedHardware/SoftwareIntroduction,(c)2000Vahid/Givargis3•Puttingitalltogether–General-purposeprocessor–Single-purposeprocessor•Custom•Standard–Memory–Interfacing•Knowledgeappliedtodesigningasimpledigitalcamera–General-purposevs.single-purposeprocessors–PartitioningoffunctionalityamongdifferentprocessortypesIntroductionEmbeddedSystemsDesign:AUnifiedHardware/SoftwareIntroduction,(c)2000Vahid/Givargis4Introductiontoasimpledigitalcamera•Capturesimages•Storesimagesindigitalformat–Nofilm–Multipleimagesstoredincamera•Numberdependsonamountofmemoryandbitsusedperimage•DownloadsimagestoPC•Onlyrecentlypossible–Systems-on-a-chip•MultipleprocessorsandmemoriesononeIC–High-capacityflashmemory•Verysimpledescriptionusedforexample–Manymorefeatureswithrealdigitalcamera•Variablesizeimages,imagedeletion,digitalstretching,zoominginandout,etc.EmbeddedSystemsDesign:AUnifiedHardware/SoftwareIntroduction,(c)2000Vahid/Givargis5Designer’sperspective•Twokeytasks–Processingimagesandstoringinmemory•Whenshutterpressed:–Imagecaptured–Convertedtodigitalformbycharge-coupleddevice(CCD)–Compressedandarchivedininternalmemory–UploadingimagestoPC•DigitalcameraattachedtoPC•SpecialsoftwarecommandscameratotransmitarchivedimagesseriallyEmbeddedSystemsDesign:AUnifiedHardware/SoftwareIntroduction,(c)2000Vahid/Givargis6Charge-coupleddevice(CCD)•Specialsensorthatcapturesanimage•Light-sensitivesiliconsolid-statedevicecomposedofmanycellsWhenexposedtolight,eachcellbecomeselectricallycharged.Thischargecanthenbeconvertedtoa8-bitvaluewhere0representsnoexposurewhile255representsveryintenseexposureofthatcelltolight.Someofthecolumnsarecoveredwithablackstripofpaint.Thelight-intensityofthesepixelsisusedforzero-biasadjustmentsofallthecells.Theelectromechanicalshutterisactivatedtoexposethecellstolightforabriefmoment.Theelectroniccircuitry,whencommanded,dischargesthecells,activatestheelectromechanicalshutter,andthenreadsthe8-bitchargevalueofeachcell.ThesevaluescanbeclockedoutoftheCCDbyexternallogicthroughastandardparallelbusinterface.LensareaPixelcolumnsCoveredcolumnsElectroniccircuitryElectro-mechanicalshutterPixelrowsEmbeddedSystemsDesign:AUnifiedHardware/SoftwareIntroduction,(c)2000Vahid/Givargis7Zero-biaserror•Manufacturingerrorscausecellstomeasureslightlyaboveorbelowactuallightintensity•Errortypicallysameacrosscolumns,butdifferentacrossrows•Someofleftmostcolumnsblockedbyblackpainttodetectzero-biaserror–Readingofotherthan0inblockedcellsiszero-biaserror–Eachrowiscorrectedbysubtractingtheaverageerrorfoundinblockedcellsforthatrow1231571421271311029923513413515711210910610813613514415910811211810912617618316111118613013213313714915412618514613113212113012714620515013012611715116018125016113412516817017117818317911212413617015514014411511224812141451461681231201171191471210144153168117121127118135991761831611111861301321330014415616113319215313813977122131128147206151131127201211551641852541651381294417317517618318818411712955CoveredcellsBeforezero-biasadjustmentAfterzero-biasadjustment-13-11-90-7-1-4-5Zero-biasadjustmentEmbeddedSystemsDesign:AUnifiedHardware/SoftwareIntroduction,(c)2000Vahid/Givargis8Compression•Storemoreimages•TransmitimagetoPCinlesstime•JPEG(JointPhotographicExpertsGroup)–Popularstandardformatforrepresentingdigitalimagesinacompressedform–Providesforanumberofdifferentmodesofoperation–ModeusedinthischapterprovideshighcompressionratiosusingDCT(discretecosinetransform)–Imagedatadividedintoblocksof8x8pixels–3stepsperformedoneachblock•DCT•Quantization•HuffmanencodingEmbeddedSystemsDesign:AUnifiedHardware/SoftwareIntroduction,(c)2000Vahid/Givargis9DCTstep•Transformsoriginal8x8blockintoacosine-frequencydomain–Upper-leftcornervaluesrepresentmoreoftheessenceoftheimage–Lower-rightcornervaluesrepresentfinerdetails•Canreduceprecisionofthesevaluesandretainreasonableimagequality•FDCT(ForwardDCT)formula–C(h)=if(h==0)then1/sqrt(2)else1.0•AuxiliaryfunctionusedinmainfunctionF(u,v)–F(u,v)=¼xC(u)xC(v)Σx=0..7Σy=0..7Dxyxcos(π(2u+1)u/16)xcos(π(2y+1)v/16)•Givesencodedpixelatrowu,columnv•Dxyisoriginalpixelvalueatrowx,columny•IDCT(InverseDCT)–Reversesprocesstoobtainoriginalblock(notneededforthisdesign)EmbeddedSystemsDesign:AUnifiedHardware/SoftwareIntroduction,(c)2000Vahid/Givargis10Quantizationstep•Achievehighcompressionratiobyreducingimagequality–Reducebitprecisionofencodeddata•Fewerbitsneededforencoding•Onewayistodivideallvaluesbyafactorof2–Simplerightshiftscandothis–Dequantizationwouldreverseprocessfordecompression115039-43-1026-831141-81-3115-73-6-222-514-111-4226-317-382-61-13-1236-23-185441337-410-217-836-11-9-420-28-2114-19-721-63312-21-5-13-11-17-4-17-41445-5-13-1015-10014-9-103-12-10-5302-50-8-2-25-3-21625-11-31-15-1-1-13-4-32-2-13-1